This invention relates to an integratable loop current detector circuit for indicating an off-hook condition of a carrier-serviced subscriber telephone.
Carrier telephone subscriber station terminal equipment is described in the U.S. Pat. Nos. 3,927,266, issued Dec. 16, 1975, Ringer Power Generator Circuit for Subscriber Carrier Station Terminal, by James A. Stewart and Neale A. Zellmer; 4,158,752, issued June 19, 1979, Carrier Subscriber Telephone Station Terminal Loop Current Booster, by James A. Stewart; 3,886,321, issued May 27, 1975, Ringer Generator for Telephone Station Terminal, by Lester Q. Krasin et al.; and 3,501,591, issued Mar. 17, 1970, Subscriber Carrier for Special Service Applications, by Lester Q. Krasin et al.
A carrier station terminal generally comprises a local battery for powering the carrier subscriber's telephone set and electrical circuitry including a transmit oscillator operating at a fixed frequency such as 28 KHz for producing a carrier developed telephone channel there. Active elements of the transmit oscillator are preferrably cut off when the subscribers handset is on-hook for conserving power in the local battery. In ones of these prior art references, an off-hook condition of the handset causes local battery current in the local subscriber loop to be sensed in a base bias resistor of a control transistor for directly turning the latter on or off. This change in the operating state of the control transistor turns on the transmit oscillator for sending a 28 KHz signal to the central office for indicating that the carrier subscriber's handset is off-hook. Such a loop current detector has limited sensitivity in that the loop current to turn the control transistor on or off must be above a minimum valve dictated by the V.sub.be of the control transistor. Also, with such a local-loop current detector there is a variable local-loop current, i.e., the local loop current there is a function of the loop resistance which is a function of the length of the subscriber's drop circuit. This may result in excessive loop current being drawn from the local battery and an unnecessarily heavy drain on the local battery for a short drop circuit between the carrier subscriber terminal equipment and the carrier subscriber's telephone. In another loop current detector for supplying a regulated loop current to the carrier subscriber's telephone, the series combination of a base bias resistor and first and second diodes is electrically connected in parallel with the series combination of a regulating or source transistor's collector-emitter path and an emitter resistor, both series strings being electrically connected across the local battery in the subscriber's local loop circuit. The junction of the base bias resistor and one diode is connected to the base electrodes of the source transistor and a control transistor that is nonconducting with the handset on-hook. If the thermal characteristics of the one diode and the source transistor's base-emitter junction are matched, then the loop current is established by the voltage on the other diode and the emitter resistor. Since the voltage drop across the second diode varies with temperature, however, this loop current detector is temperature dependent. Also, the maximum loop length for which this circuit will operate is limited since the regulating transistor's collector must be operated out of saturation in order to provide the requisite base current in the first named resistor for turning on the control transistor. An object of this invention is the provision of an improved loop current detector.